8/05/2009 @ 6:00AM

Comets: Cosmic Life Preservers

Comets have been blamed for a lot of death and destruction over the years. Showers of them are thought to have triggered at least one and maybe even several of the major and minor mass-extinction events that have wiped out millions of species in the 4 billion years since life on earth first began.

But new research from the University of Washington defends comets, those gassy balls of ice and dust that swing by earth every once in a while to show off their tails. Doctoral student Nathan Kaib ran some data that suggests that the number of comets available to rain down on earth from time to time has been hugely overestimated.

“Comets are a convenient explanation for extinction events,” says Kaib, whose work was published in the online edition of the journal Science. But his calculations argue that comets may have caused only one minor extinction event in 4 billion years.

While Kaib has been working for the comets’ defense, Chandra Wickramasinghe, director of the Cardiff Centre for Astrobiology at Cardiff University in Wales, has been comet’s PR man: He has been gathering evidence to support his long-held belief that comets are not only not menacing, they are responsible for life on earth.

Wickramasinghe, who has spent his career helping modernize the ancient theory of panspermia, the idea that seeds of life are scattered throughout the universe, published a study recently that he says shows that comets harbored oceans in which life could have flourished.

“It is almost a certainty in the early days of comets that they consisted of huge oceans of warm liquid water,” he says. “Even if a handful of bacteria spores were incorporated in these huge oceans, they could have reproduced rapidly and the oceans would soon be swarming with microbiology.”

Comets were formed along with the solar system in the aftermath of a nearby supernovae explosion. Energy from the star’s explosion spurred the gas and dust in our neighborhood to coalesce into our sun and the planets about 4.5 billion years ago.

The rocky stuff that didn’t coalesce into planets formed asteroids that hang out between Mars and Jupiter. The gassy stuff that didn’t coalesce into planets formed the comets that live beyond Neptune, in a region called the Kuiper belt. And the gassy stuff that nearly escaped the sun’s orbit completely formed comets in what is known as Oort cloud, a sphere of comets surrounding us at the very furthest reaches of our solar system, 300 billion miles to 20 trillion miles from the sun.

We’ve long had an idea about the composition of comets by studying the wavelengths of light reflected off them. In 2004, NASA’s Stardust Mission collected material from a comet’s coma, the dust cloud around the comet that produces the comet’s tell-tale tail. Then, on July 4, 2005, the NASA program Deep Impact got a closer look–it smashed into a passing comet and studied the resulting explosion.

Stardust and Deep Impact revealed complex organic material as well as clays, which Wickramasinghe says almost certainly need liquid water to form. Using data from Stardust and Deep Impact and other recent observations, Wickramasinghe calculates that radiation from a radioactive form of aluminum produced in the supernovae melted water that formed oceans that lasted a million years. “You can work out how the comets behaved since they were born,” he explains.

For most of earth’s history, it has been protected from comet strikes by Jupiter and Saturn. The planets are so big that when comets are pushed into an orbit that would send them toward the inner solar system, where earth sits, the giant planets’ gravity deflects them safely askew.

About 4 billion years ago, however, the orbits of the new planets, which hadn’t yet settled into a stable path, are thought to have shifted suddenly, causing gravitational havoc. This is thought to have sent a deluge of comets and asteroids to earth called the Late Heavy Bombardment. Coincidently–or not, says Wickramasinghe–just after this period is when the first chemical evidence of life shows up on earth.

Wickramasinghe argues that life is extremely difficult to start and, given the increasingly inhospitable places researchers have found life lately, also extremely difficult to destroy. He says it is possible that life formed on a planet around the star that exploded to help form our solar system. Life could have survived the star’s death in a spore-like form, and flourished in the oceans inside comets that later smashed into earth.

“This appeared to be a way-out theory 20 years ago, but it’s become a real possibility,” Wickramasinghe says. “We do not have any knowledge about how to turn non-life into life. Once that transformation took place, somewhere in a galactic context, it spread inexorably because bacteria have all these marvelous properties. Life is a continuous process that’s unstoppable.”

Next Wickramasinghe hopes to find a way to collect material from a comet without destroying whatever life or life-creating molecules might be on board. “So far, we brought back stuff with telltale signs of life, but not life,” he says.

Some argue that after the Late Heavy Bombardment periodic showers of comets (perhaps sent earthward from the Oort cloud by gravity from a passing star) caused periodic extinction events, where large numbers of species disappeared in a short period of time.

It’s plausible–there is good evidence that a giant comet or asteroid crashed into earth 65 million years ago and led to the extinction of the dinosaurs. But evidence for comet showers is weak, and the University of Washington’s Kaib suggests that’s because these comet showers probably never happened.

Comets like Hale-Bopp, which was visible from earth in 1997, were thought to come from the desolate, furthest reaches of the Oort cloud. These far-off comets have such unstable orbits that even weak gravitational nudges can bump them off line and toward the sun.

Kaib simulated the evolution of comet clouds in the solar system for 1.2 billion years and discovered that comets can also come from the inner Oort cloud, which was thought to be far denser with comets. He found, to his surprise, that the gravity of Jupiter and Saturn can deliver an “energy kick” to comets from the inner Oort cloud, sending them into orbits that make them look like they are in fact coming from the outer Oort cloud.

“They can have weak interactions with planets and hop,” says Kaib. “This gives people the impression that they came from far away.”

Kaib’s calculations increase the probability that a comet from the Oort cloud could reach earth, because now it seems comets can come from any part of the cloud. But it significantly decreases the number of comets thought to be in the cloud and drastically shrinks the possibility of a mass-extinction-causing comet shower. (A shower from the Kuiper belt has long seemed impossible.)

Richard A. Muller, a physics professor at the University of California, Berkeley, advanced the theory that an unseen dark star orbiting nearby, which he dubbed Nemesis, could have caused periodic comet showers. He praises the modeling work done by Kaib, but says that his conclusions are unjustified.

Bright comets visible to the naked eye only come around once every few years, but there are always comets flying around out there, often observable with binoculars or a telescope. You might want to give them a wave. That could be a comet full of relatives.

“The earth is intimately connected with the rest of the universe through comets,” Wickramasinghe says. “We are connected to the much bigger universe physically and biologically.”